Ytterbium catalysis

As anticipated from the complexation experiments, reaction of 4.42 with cyclopentadiene in the presence of copper(II)nitrate or ytterbium triflate was extremely slow and comparable to the rate of the reaction in the absence of Lewis-acid catalyst. Apparently, Lewis-acid catalysis of Diels-Alder reactions of p-amino ketone dienophiles is not practicable. 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

Catalysis of the aminolysis of epoxides by lanthanide triflates (ytterbium, neodymium and gadolinium trifluoromethanesulphonate) has been reported (e.g. equation 26)68. 

Active catalyst species or catalysis intermediates can often be trapped by stoichiometric reactions of the precatalyst with the substrate. The following example describes the successful isolation of such an intermediate with participation of Ln-O cr-bonds. Reduction processes mediated by low oxidation states of the lanthanide elements are of special interest in organic synthesis [256]. One of the most intensively studied reactions is the stoichiometric reduction of arylketones by rare earth metals ytterbium and samarium [277]. Thus formed dianions possess high nucleophilic character and excess lanthanide metal can even accomplish complete cleavage of the C-O double bond (Scheme 36). 

Ding, R., K. Katebzadeh, L. Roman, K.-E. Bergquist and U.M. Lindstrom, Expanding the Scope of Lewis Add Catalysis in Water Remarkable Ligand Acceleration of Aqueous Ytterbium Triflate Catalyzed Michael Addition Reactions, Journal of Organic Chemistry, 71, 352-355 (2006). 

The same differential behavior can be observed with amine nucleophiles. For example, calcium triflate promotes the aminolysis of propene oxide 84 with benzylamine to give 1-(A -benzyl)amino-2-propanol 85, the result of attack at the less substituted site <03T2435>, and which is also seen in the solventless reaction of epoxides with heterocyclic amines under the catalysis of ytterbium(III) triflate <03SC2989>. Conversely, zinc chloride directs the attack of aniline on styrene oxide 34 at the more substituted carbon center <03TL6026>. A ruthenium catalyst in the presence of tin chloride also results in an SNl-type substitution behavior with aniline derivatives (e.g., 88), but further provides for subsequent cyclization of the intermediate amino alcohol, thus representing an interesting synthesis of 2-substituted indoles (e.g., 89) <03TL2975>. 

Considerable rate enhancements have been observed when water is used as solvent compared with alcoholic or hydrocarbon media for Mannich reactions, i.e. condensations of ketones with secondary amines in the presence of formaldehyde [65]. Allylsilanes [66] and allylstannanes [67] in aqueous media were used in organic synthesis under Mannich-like conditions. More recently, Kobayashi reported the catalysis of the reaction of vinyl ethers with iminium salts by ytterbium triflate in tetrahydrofuran-water mixtures [68]. 

The ring opening of epoxides with amines was investigated under several conditions including high pressure and catalysis by ytterbium triflate (Scheme 10.14) [43]. It was found that the combination of pressme and Yb(OTf)3 catalysis was... 

The use of Montmorillonite clay, a very efllcient acidic catalyst, allows a-alkylation of / -substituted indoles ytterbium triflate can also be used to catalyse such alkylations. This efficient catalysis contrasts with the different, but very instructive, reaction pathway followed when mesityl oxide and 1,3-dimethylindole are combined in the presence of sulfuric acid - electrophilic attack at the already substituted /3-position is followed by intramolecular nucleophilic addition of the enol of the side-chain ketone to C-2. ... 

BiCfr [34b] proved to be a better catalyst than scandium [76] or ytterbium triflates [77] for this rearrangement which is also catalyzed by Bronsted acids [78]. Other functionalities such as Boc, allyl, and propargyl are compatible with bismuth catalysis (Section 11.2.6). 

Greeves e( al. reported that lanthanide triflates were effective for allylation of carbonyl compounds (20). Five mole percent of Yb(OTf)3 catalyzed the allylation of a series of aldehydes in good yield (Scheme 13.9) [20a]. Later they found a significant rate of acceleration by the Bronsted add in allylation of aldehydes with allyltributyltin catalyzed by ytterbium triflate (Table 13.9) [20b]. Recently, the concept of combined Lewis acid-Bronsted catalysis has attracted much attention from the chemical community because it often gives rise to significant improvement... 

Ding, R., Katebzadeh, K., Roman, L., Bergquist, K.-E. and Lindstrom, U.M., Expanding the scope of Lewis add catalysis in water remarkable Ugand acceleration of aqueous ytterbium triflate catalyzed Michael addition reactions, /. Org. Chem., 2006,71,352-355. 

A convenient and efficient synthesis of the a-pyrone kawain was also described that used 1.7 mol% Eu(fod) to give a 75% isolated yield (based on diene) (Castellino and Sims, 1984b). Catalysis by the corresponding complex of ytterbium, a stronger Lewis acid than europium, gave an 84% yield, while use of a stoichiometric amount of zinc chloride resulted in a 78% yield. 

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